1. FIELD OF THE INVENTION
The invention relates to a method and apparatus for effecting the completion of a subterranean well bore which is initially drilled in a substantially vertical direction and traverses a curve to produce a non-vertical portion, which may be horizontal, traversing a production formation.
2. SUMMARY OF THE PRIOR ART
For many years the desirability of utilizing a subterranean well bore having a non-vertical or horizontal portion traversing a production formation has been known and appreciated in the prior art. Laterally directed bores are drilled radially, usually horizontally, from the primary vertical well bore, in order to increase contact with the production formation. Most production formations have a substantial horizontal extent and, when conventional vertical well bores are employed to tap such production formations, a large number of vertical bores must be employed. With the drilling of a well bore having a non-vertical or horizontal portion traversing the production formation, a much greater area of the production formation may be traversed by the well bore and the total field drilling costs may be substantially decreased. Additionally, after a particular horizontal well bore has produced all of the economically available hydrocarbons, the same vertical well bore may be redrilled to establish another horizontal portion extending in another direction and thus prolong the utility of the vertical portion of the well and increase the productivity of the well to include the total production formation.
As stated in SPE paper #16929 presented at the meeting of the Society of Petroleum Engineers held in Dallas, Tex. on Sept. 27-30, 1987:
"The application of well servicing equipment to that of horizontal well completions presents specific design considerations that do not arise in the norm of conventional well completions. Although tubing conveyed perforating has been most suited to highly deviated wells and long intervals, horizontal well designs must consider other factors. Short radius turns, long measured depths and extended pay intervals give rise to mechanical complications while running the guns, obtaining an underbalance, detonating the charges, and removing the guns from the well . . . PA1 As the techniques employed during horizontal drilling projects become more refined, the next major area of concern becomes that of effecting an efficient completion. Many of the problems encountered in horizontal drilling were not exposed until actual drilling began, thus many of the drilling refinements were achieved through trial and error. The first horizontal wells to be drilled were completed without casing in the productive zone. Slotted liners were then used in wells having greater radius of curvatures and today unperforated casing is being used and even cemented on occasion. Because of lower drilling costs and extended reach capabilities, many operators are favoring long radius (1000-2800 feet) turns or drilling wells with long ramp sections, at 40-50.degree., that build the horizontal in the pay zone. It is these wells that are being completed today with somewhat conventional, but modified, techniques. In the future, completion operations are likely to uncover unforseen complications. Several of the drilling improvements will most likely be adapted to completion operations, some of which will be the evolution of flexible bottom hole assemblies and specialized centralizing techniques, especially for wells with short radius turns".
One of the completion techniques that has not been worked out, particularly for wells having medium radius turns on the order of about 10.degree.-90.degree. per 100 feet of well length, is the design, insertion and operation of gravel packing or other Sand Control equipment. Wells incorporating such medium radius curvature between the horizontal and vertical portions of the well bore have been completed only by utilizing the open hole technique or inserting a slotted liner.
U.S. Pat. No. 4,553,595 to Huang et al proposed that gravel packing be accomplished in two distinct steps. In the first step, the horizontal segment of the well bore is provided with a foundation layer of unconsolidated gravel supplied through a flexible tube, hence with no control of the depth of the layer. A perforated liner is then introduced into the well resting on the initial layer of gravel. Those skilled in the art will recognize that "gravel" used in gravel packing of a well, as well as the material referred to herein in "pre-packed" screens, can be coarse sand, glass beads, solid polymeric like substances, and the like, and can generally be defined as solid particulate matter which blocks the entry into the production tubing of produced sand and other solids but permits the flow into the production tubing of the production fluids.
The second phase of the gravel pack is achieved by introducing a gravel slurry through flexible hoses attached to the exterior of the slotted liner, such slurry being deposited on the foundation bed and hopefully building up around the liner. The disadvantages and shortcomings of this approach will be readily apparent to those skilled in the art.
Gravel packing of conventional wells is efficiently accomplished by gravel packing apparatus of the type disclosed in U.S. Pat. No. 3,987,854 to Callihan et al. The apparatus employed in this patent incorporates a hydraulically operated packer, which is connected to the top of a screen section. The packer is provided with conventional left hand threads which in turn are connected to a left hand threaded nut carried on the top end of a cross-over tool which is secured to the bottom end of a tubing string.
This conventional type of gravel packing apparatus is unsuited for the gravel packing of horizontal well bores having a significant longitudinal extent on the order of 1000-2000 feet, and more. In the first place, to insert this apparatus through the radius portion of the well bore, it may be necessary that the apparatus be rotated, which rotation will of course be in a clockwise direction, as is conventional. The resistance to passage of the apparatus through the short radius curved sections of the well bore is sufficiently high as to allow clockwise rotation of the left hand threaded nut relative to the packer. Thus, the packer and nut would become disengaged upon the application of excessive torques required to rotationally insert the gravel packing tool into the well.
An even more important deficiency lies in the fact that the introduction of the gravel packing fluid is conventionally accomplished by flowing the gravel containing fluid externally through the annulus defined between the screens and the well bore. With a well bore having a non-vertical, and particularly a horizontal extent of 1000 to about 2000 feet, there is no assurance that the gravel contained in the gravel packing fluid will not build up around centralizers and bridge across the annulus long before it reaches the end of the 1000-2000 foot length of screen traversing the horizontal production formation.
Any attempt to utilize conventional centralizers to center the multitude of screens relative to the horizontal well bore also creates additional problems. All conventional centralizers are subject to destruction by rotation of the tubing string upon which they are carried, particularly when compressed between the wall of the well bore or casing and the tool string. Thus, the employment of conventional stabilizers, which are essential for locating the screens axially aligned within the well bore, is effectively prohibited by the required rotation of the screens during the process of inserting the tool string through any radius curved portion of the well bore. These are only a few of the problems that must be solved if the industry goal of achieving efficient production from well bores having non-vertical or horizontal portions of substantial longitudinal extent traversing production formations, is to be accomplished.
It is accordingly an object of this invention to provide a method and apparatus for effecting the completion of well bores having a substantial length non-vertical portion traversing a production formation, and particularly to effect the gravel packing of such non-vertical or horizontal well bore portions.